Reaction with which of the following reagents give benzaldehyde from toluene ?

Question

Accepted Answer

To convert toluene to benzaldehyde, the reagent $CrO_{2}Cl_{2}$, also known as chromyl chloride, is used. This reaction is known as the Etard reaction. Here's a detailed explanation of why Option 1 is correct and why the other options are not suitable for this transformation:

1. **Option 1: $CrO_{2}Cl_{2}$ (Chromyl Chloride)**
   - Chromyl chloride is specifically used in the Etard reaction to oxidize the methyl group of toluene to form benzaldehyde. The reaction involves the formation of a complex between chromyl chloride and toluene, which upon hydrolysis yields benzaldehyde. This method is selective for the oxidation of the methyl group to the aldehyde without further oxidation to the carboxylic acid, making it the correct choice for this transformation.

2. **Option 2: $K_{2}Cr_{2}O_{7}$ (Potassium Dichromate)**
   - Potassium dichromate is a strong oxidizing agent that typically oxidizes primary alcohols to carboxylic acids and secondary alcohols to ketones. In the case of toluene, $K_{2}Cr_{2}O_{7}$ would likely oxidize the methyl group all the way to benzoic acid rather than stopping at the aldehyde stage. Therefore, it is not suitable for selectively producing benzaldehyde from toluene.

3. **Option 3: DIBAL-H (Diisobutylaluminium Hydride)**
   - DIBAL-H is a reducing agent, not an oxidizing agent. It is commonly used to reduce esters, nitriles, and other functional groups to aldehydes or alcohols. Since the task is to oxidize toluene to benzaldehyde, DIBAL-H is not applicable for this transformation.

4. **Option 4: $CH_{3}COCl/$ anhydrous $AlCl_{3}$ (Friedel-Crafts Acylation)**
   - This combination is used for Friedel-Crafts acylation reactions, which involve the introduction of an acyl group into an aromatic ring. This reaction would not convert toluene to benzaldehyde; instead, it would introduce an acyl group onto the aromatic ring, resulting in an acetophenone derivative rather than benzaldehyde.

In conclusion, the use of chromyl chloride ($CrO_{2}Cl_{2}$) in the Etard reaction is the correct method for selectively oxidizing toluene to benzaldehyde, making Option 1 the correct answer.